West Yorkshire Police confirmed they were called to Paradise Bar in the early hours of Sunday morning over the attack, which left a 17-year-old male needing hospital treatment.
Four males, including three teenagers, have been arrested over the assault.
A cordon remained in place around the bar throughout Sunday.
A spokeswoman for West Yorkshire Police said: "Police were called at 4.15am yesterday morning (28/07) to a report of an assault outside a bar, at the junction of Sheepscar Street South and Roundhay Road.
"A 17-year-old male sustained a serious slash wound and was taken to hospital, where he remains in a serious condition.
"Four men, aged 24, 19, 18 and 17, were arrested and have been released while enquiries continue.
"Enquiries remain ongoing by Leeds CID. Anyone with information is asked to contact police online via www.westyorkshire.police.uk/contact-us or by calling 101, quoting crime reference 13190383443."